Dixon Systems NM-250 MKII Manuel utilisateur
Dixon Systems
Model NM-250 MKII
Owner's Manual
Dixon Systems NM-250 MKII
Introduction
The NM-250 MKII newsroom mixer is designed for newsroom work-stations and
contains almost all the features of a full stereo broadcast console in a two rack-space
unit. The NM-250 MKII retains all the features of the original NM-250 but adds a
number of features and improvements.
The NM-250 MKII improvements include new microphone input stages that can be
easily changed to line level with no component changes. A simple jumper selects
either ‘mic’ or ‘line’. Tally LEDs have been added to the two talk back push-buttons to
indicate who is calling. Both a ‘Start’ and ‘Stop’ pulse are now provided for the
telephone input and the computer input. This makes interfacing with a wider variety of
hybrids much easier.
Overall, the NM-250 MKII features two greatly improved microphone/line inputs with
phantom power for condenser microphones, a mix-minus bus for feeding a telephone
hybrid, an input for the hybrid return, an interface for the computer sound card, a
miniature jack front panel input for cassette/DAT machines, and two balanced +4 line
inputs. In addition, the unit features a cue system as well as a talk back system with
two sends and returns. It contains a built in headphone amplifier and an output for a
stereo monitor amplifier. (See information about Dixon Systems’ RM-35.) A front panel
control permits the monitoring to be switched between the output of the mixer and an
external input, usually an off-air feed.
To achieve the best performance possible, low noise 1% metal-film resistors are
used throughout the audio path. A toroidal transformer is used in the power supply to
ensure quiet, cool and clean operation.
Dixon Systems NM-250 MKII
Unpacking and Inspection
Carefully examine the contents of the shipping carton for any sign of physical
damage which could have occurred in transit.
IF DAMAGE IS EVIDENT, DO NOT DESTROY ANY OF THE
PACKING MATERIAL OR CARTON. IMMEDIATELY NOTIFY
THE CARRIER OF A POSSIBLE CLAIM FOR DAMAGE. SHIPPING
DAMAGE CLAIMS MUST BE MADE BY THE CONSIGNEE.
Environmental Considerations
The NM-250 MKII will operate satisfactorily over a wide range of ambient
temperatures. If installed in an area with high heat producing equipment, adequate
ventilation is necessary to prolong the life of components.
Wiring for local power line Conditions
The units have been pre-wired at the factory for 110 VAC operation, as indicated on
the shipping cartons. To comply with existing electrical codes, each unit is supplied with
a three-wire AC power cord with the ground pin connected to the chassis.
Never replace the fuse with anything other than a 1/3 amp Slo-Blo fuse.
General
The NM-250 MKII is ruggedly constructed and should provide years of trouble-free
use with normal care. Occasional cleaning with a damp cloth is recommended for the
front panel, and should dust collect inside the unit, disconnect it from AC power, remove
the top cover, and use a small vacuum to gather up the dirt. Be careful not to dislodge
any of the internal connectors. No other special preventive maintenance is required.
All parts are conservatively rated for their application, and all ICs are socketed for
ease of replacement. Workmanship meets the rigid standards professional users
expect.
Dixon Systems NM-250 MKII
Performance Specification
Line Inputs
Balanced Inputs
Source impedance 600 Ohms
Input impedance 40 kOhms
Input level Nominal +4dBm
Maximum input level +28 dBm
Unbalanced Inputs
Source impedance < 2 kOhm
Input impedance 20 kOhms
Input level Nominal -10 dBu
Distortion & T.H.D. < .01% 20-20 kHz
Typical: .005% ( +4 in/ +4 out)
Noise Better than -80dB (Ref. 0 dBu)
Intermod. (4:1) < .005% (Typical: .003%)
Frequency Response Better than +/- .5 dB
20 Hz-20 kHz
Microphone Inputs
Source impedance 150 Ohms
Input type Active Balanced
Input level Nominal -56 dBu
Distortion & T.H.D. < .01% @ G=100 (Full range)
Frequency Response Better than +/- .5 dB
30 Hz-15 kHz
Master Outputs
Output impedance 60 Ohms
Active balanced
Output level + 4 dBm (as shipped) Adjustable
Maximum Output level +22 dBm
Cross Talk Better than -60 dB
Other Outputs
Unbalanced Output (RCAs) Nominal -10 dBu
Rear panel Adjustable
Monitor output Active Balanced +4 db
Front panel gain adjust
Talkback Outputs Active balanced
Nominal +4 dBm (Adjustable)
Headphone Amp 350 mw into 8 Ohms
Front panel gain adjust.
Front panel jack
Dixon Systems NM-250 MKII
NM-250 MKII Simplified Block Diagram
Dixon Systems NM-250 MKII
Microphone Inputs
The NM-250 MKII has three low-Z microphone inputs. They are identical, except
that Mic 1 is also used as part of the Talkback send system and the third input is
normally configured as a line input return from the telephone hybrid. Input connectors
are XLR type and are located on the rear panel. Inputs one and two both have phantom
power to allow the use of high quality condenser microphones if desired. Gain is fixed,
but most microphones used in today’s facilities should have adequate output for the
mixer. A simple computer style jumper located on the board near the input IC
determines if the stage works at mic or line level. Either microphone can mute the
monitor output. The mute function is programmed via jumpers located on the rear of the
circuit board mounted on the front panel. When shipped, both microphones are set to
mute.
Microphone 1 should be used by the operator. The second microphone can be used
in a booth or studio if desired, and has a Cue switch to permit monitoring.
Dixon Systems NM-250 MKII
Talkback Send System
The talkback system in the NM-250 MKII allows communication to and from two
remote locations. A typical situation might be between the newsroom, a studio or booth,
and the control room. Talkback # 1 has been designed to permit the use of a TCC or
single pair for both send and receive. This is most often used when the NM-250 MKII is
acting as a remote mixer. New in the ‘250 MKII are tally lights that indicate who is
calling. These are controlled by simple ground starts located on the rear panel.
Audio from the first microphone pre-amp is sent to the talkback-send op-amp where
additional gain is added. Output level is a nominal +4 dBm (balanced) and gain is
adjusted via the trim pot on the main PC board. This trim pot sets the level for both
send outputs. When a talkback button on the front panel is pressed, it activates one of
the two talkback relays and audio is fed to the rear panel connector. At the same time,
the monitor mute relay is activated and a contact closure is provided for signalling the
talkback-receive station.
Dixon Systems NM-250 MKII
Talkback Receive System
The NM-250 MKII has two talkback receive inputs. When a signal is sent to either of
these inputs, it is applied to one half of a 5532 op-amp and the receive level is adjusted
via one of two trim pots located on the main PC board. Nominal receive level is +4 dBm
(balanced). The signal is then sent to the monitor summing-amplifier where it is added
to one side of the monitor system. A ground start is available on the rear connectors for
‘tally’ for each talkback input as well as a ground start for the DIM function. Simply
grounding the appropriate pin is all that’s required.
As indicated earlier, talkback # 1 can be used with a single twisted pair, (sometimes
referred to as a TCC) most often when the NM-250 MKII is used as a remote mixer.
Dixon Systems NM-250 MKII
Telephone Input
The output of the telephone hybrid normally goes to the third input of the NM-250
MKII. It is identical to the microphone inputs, except that the ‘mic/line’ jumper is not
installed. Nominal input level is 0 dBu. The NM-250 MKII also provides a contact
closure to ground (on the remote control connectors) whenever the telephone channel
is turned on, eliminating the need to activate more than one switch when an interview is
being conducted. As well, this channel can be configured to mute the monitor output of
the mixer, should this be required. New in the ‘MKII are ‘on’ and ‘off’ open collectors to
control the hybrid. These provide a pulse to ground of approximately 100ms.
Some customers are using the NM-250 MKII as a remote mixer. If desired, and a
phone channel is not required, this input can also be and used as a third microphone
input. It should be noted that the two 6K81 resistors for phantom power are not
installed in this channel, and that the 20K resistor feeding the mix-minus bus has also
been removed, preventing feedback when used with a hybrid.
Dixon Systems NM-250 MKII
Computer Interface
The NM-250 MKII interfaces easily with computer sound cards. Usually, some kind
of unbalanced to balanced unit (such as Dixon Systems “LineMatcher” model LM100B)
is required to connect a sound card to professional equipment. This is not necessary
with the NM-250 MKII. There are four RCA-type connectors on the rear panel of the
unit, two as inputs coming from the sound card and two which send audio to the sound
card. They are clearly marked.
As seen in the schematic below, the output from the sound card is applied to a
differential input op-amp, a 5532. Nominal input level is -10 dB. The level coming from
the sound card should be set so that the control on the front panel is between one and
two o’clock. This will ensure adequate headroom.
The NM-250 MKII now provides a pulse to ground for both ‘start’ and ‘stop’. These
can be used to start and stop the computer playback. Pulse duration is about 100 ms.
Each time the channel is turned on or off it generates a pulse. The connections are
made via the connectors on the rear panel.
The output to the sound card comes directly from the output stage of the mixer. For
more information, see the output section of this manual.
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